The cells involved in immune responses at mucosal surfaces will be delineated and their biological functions determined. The cells involved in host defenses against bacterial infections of the gut and respiratory tracts of rodents will receive special attention. The modulating effects of the normal intestinal flora on the systemic immune responses of gnotobiotic animals will be determined. Characterization of the cells in the rodent Peyer's patches will be studied using current technology. Control of secretory IgA production and delayed hypersensitivity responses at mucosal surfaces will be studied.